This invention relates to a method for applying a liquid coating compound to a long, flexible support or web being conveyed.
A conventional coating method which has been extensively employed for applying a liquid coating compound or coating liquid to a web in a method using a so-called "multilayer slide bead coating apparatus" is disclosed, for example, in U.S. Pat. No. 2,716,791 issued to Russell et al. According to this conventional method, plural coating liquids flow down a slide surface falling against the web being conveyed to form a bead thereon. With a coating apparatus of this type, it is essential to maintain the bead stable. However, as the coating speed is increased, it usually becomes difficult to maintain the stability of the bead.
U.S. Pat. No. 2,681,294 discloses a technique in which means for reducing the pressure on the lower surface of the bead is used to provide a difference in pressure between the upper surface and the lower surface of the bead thereby to maintain the bead stable. However, the stability of the bead depends not only the balance in pressure between the upper and lower surfaces of the bead. That is, the widthwise balance of the bead, namely the stability at both ends of the bead, must be also taken into consideration. This can by understood from the fact that, as the coating speed is increased, breakage of the bead begins with the two ends thereof.
In order to prevent the bead from being broken in this way, a method has been proposed in the art in which the flow rate of coating liquid at the two ends of the bead is made higher than that at the remaining portions of the bead. However, this method is not suitable for most applications because with it the thickness at both edge portions of a layer is greater than that of the remaining portions thereof. Also, it may be difficult to completely dry both edge portions with the same process.
Accordingly, an object of the invention is to provide a coating method in which all of the above-described difficulties accompanying a conventional method have been eliminated and in which breakage of both end portions of the bead is prevented without coating both end portions thicker and yet in which a high speed coating operation is possible.